In order to respond to a change for cordless electronic equipment in recent years, nonaqueous cells, such as lithium cells (lithium primary cells) and lithium ion secondary cells, have attracted attention as power sources because these cells are light in weight and give a high electromotive force and a high energy. For example, cylindrical lithium secondary cells are manufactured in large amounts because these cells are used for portable telephones, notebook type personal computer, and others. The production quantity thereof has been increasing year by year. Furthermore, nonaqueous cells also have attracted attention as energy sources for next-generation electric vehicles. The requirements for cells have been increased to output a higher power by a further restraint of the electrical resistance thereof.
A typical electrolytic solution used for such nonaqueous cells has been known such that a solution in which LiPF6, LiCF3SO3, LiClO, LiBF4 or the like is dissolved in an organic solvent, which will be detailed later. When an abnormal current flow is generated in a cell containing the above electrolytic solution due to a short circuit or a wrong connection between, for example, positive and negative electrodes, there is a possibility that the cell produces remarkable increase in temperature in connection with the abnormal current flow. When the cell causes such an increase in temperature, an attempt has been proposed to make a portion of a separator of the cell melt so as to prevent such a short circuit or wrong connection (see, for example, Patent Document 1).
Hitherto, as a material constituting a separator for a nonaqueous cell, glass fibers and regenerated cellulose have been studied because glass fibers have heat resistance and endurance and are capable of being fine fibers and regenerated cellulose is capable of being refined or beaten. However, glass fibers lack self-bondability, and are unable to be expected to have a high strength caused by entanglement between fibers. Accordingly, studies on various binders have been made in order to improve the strength of aggregate of glass fibers (see, for example, Patent Documents 2 to 5). On the other side, although regenerated cellulose can gain strength by advancing the refinement or beating thereof, investigations have been made about balance between the strength and other performances of aggregate of regenerated cellulose fibers (see, for example, Patent Documents 6 and 7).